Python how to write to a binary file?

Question

This is exactly what bytearray is for:

newFileByteArray = bytearray(newFileBytes)
newFile.write(newFileByteArray)

If you're using Python 3.x, you can use bytes instead (and probably ought to, as it signals your intention better). But in Python 2.x, that won't work, because bytes is just an alias for str. As usual, showing with the interactive interpreter is easier than explaining with text, so let me just do that.

Python 3.x:

>>> bytearray(newFileBytes)
bytearray(b'{\x03\xff\x00d')
>>> bytes(newFileBytes)
b'{\x03\xff\x00d'

Python 2.x:

>>> bytearray(newFileBytes)
bytearray(b'{\x03\xff\x00d')
>>> bytes(newFileBytes)
'[123, 3, 255, 0, 100]'

Use struct.pack to convert the integer values into binary bytes, then write the bytes. E.g.

newFile.write(struct.pack('5B', *newFileBytes))

However I would never give a binary file a .txt extension.

The benefit of this method is that it works for other types as well, for example if any of the values were greater than 255 you could use '5i' for the format instead to get full 32-bit integers.

As of Python 3.2+, you can also accomplish this using the to_bytes native int method:

newFileBytes = [123, 3, 255, 0, 100]
# make file
newFile = open("filename.txt", "wb")
# write to file
for byte in newFileBytes:
    newFile.write(byte.to_bytes(1, byteorder='big'))

I.e., each single call to to_bytes in this case creates a string of length 1, with its characters arranged in big-endian order (which is trivial for length-1 strings), which represents the integer value byte. You can also shorten the last two lines into a single one:

newFile.write(''.join([byte.to_bytes(1, byteorder='big') for byte in newFileBytes]))

To convert from integers < 256 to binary, use the chr function. So you're looking at doing the following.

newFileBytes=[123,3,255,0,100]
newfile=open(path,'wb')
newfile.write((''.join(chr(i) for i in newFileBytes)).encode('charmap'))

You can use the following code example using Python 3 syntax:

from struct import pack
with open("foo.bin", "wb") as file:
  file.write(pack("<IIIII", *bytearray([120, 3, 255, 0, 100])))

Here is shell one-liner:

python -c $'from struct import pack\nwith open("foo.bin", "wb") as file: file.write(pack("<IIIII", *bytearray([120, 3, 255, 0, 100])))'

Use pickle, like this: import pickle

Your code would look like this:

import pickle
mybytes = [120, 3, 255, 0, 100]
with open("bytesfile", "wb") as mypicklefile:
    pickle.dump(mybytes, mypicklefile)

To read the data back, use the pickle.load method